1. Field of the Invention
The field of the invention is digital controllers for controlling a machine, an assembly line or a process. More particularly, the invention relates to Input/Output (henceforth "I/O") modules for controlling output devices and monitoring input devices on a controlled machine or system.
2. Description of the Background Art
Programmable controllers are employed in many industrial and commercial applications to control the operation of various machines and processes. They fall into the broad category of real-time sampled data systems. In a certain maximum time, inputs are read, controlling equations are applied, and actuator outputs are provided. Besides controlling switching devices, these controllers also control closed loop servo devices by generating analog output signals and by reading encoded feedback signals and comparing actual position to desired position.
Controllers generally have a modular construction with some type of system processor module and a number of I/O modules for connection to the I/O devices on a controlled machine or process. The modules are often supported in a chassis and plugged into a backplane circuit board for electrical interconnection and communication.
It is typical to provide an isolation interface in I/O modules to physically and electrically isolate 120-volt alternating current (henceforth "AC") signals or 24-volt direct current (henceforth "DC") signals, for example, on the machine side of the controller, from the 5-volt logic-level signals within the controller electronics. This electrical isolation is typically accomplished with optical coupling circuits. With optical coupling, an overcurrent or fault on the machine side of the I/O module is isolated from the controller electronics.
Techniques for utilizing isolation interfaces in I/O modules have been previously disclosed in Maskovyak et al., U.S. Pat. No. 4,771,403, issued Sep. 13, 1988, and in Pepera et al., U.S. Ser. No. 07/580,716 now abandoned, both of which are assigned to the assignee of the present invention.
In Maskovyak et al., data is transmitted across an isolation interface as serial data, but in only one direction in an individual I/O module. In Maskovyak et al., four bits of address are transmitted in parallel across the isolation interface to retrieve each bit of serial data. In total, the isolation interface is six bits wide with five bits being needed for one-way data transfer.
In Pepera et al., a stream of serial data bits is associated with two bits for timing signals, so that that three bits are required to transfer data in one direction. Pepera et al. has been extended to two-way communication with three parallel bits being needed for transfer of serial data in each direction.